A Riddle

Fifteen score and more I weigh
When reckoned by the brood of Re
But less than one score you would quoth
If counting true by rule of Thoth.

I was born by Father Greg
When Famous Bill to Anne bent leg
And hail�d by Rome so full of pride
But on my birth great Caesar died.

I keep account and deft compute
For every man, genteel or brute
But don�t correct me when I sum
And twelve of thirty�s less than one

I�ve been named for Gods and Kings
And kin cavort with Maya and Ming
But sheep and fish define my frame
Tell me now what is my name?

Answer:calendar